Gas turbine engines are widely used to propel aircraft, to generate electricity, and to pump fluids. There is an increasing requirement for higher performance and better fuel economy in such engines. At this point the most readily available approach to increase efficiency and power output is to increase operating temperature. However, many gas turbine engine components currently operate very near their melting points, in fact many gas turbine engine components are internally cooled.
In recent years it has become fairly common to use ceramic insulating coatings on the external (heated) surfaces of gas turbine components, particularly air cooled gas turbine components, to permit operation at higher temperatures.
There is a wide variety of thermal barrier coatings used in gas turbines, they are all ceramic based, and most use yttria stabilized zirconia as the ceramic. Ceramic coatings can be applied by vapor deposition techniques (see for example U.S. Pat. Nos. 4,321,311; 5,238,752 and 5,514,482) or by thermal spray techniques (see for example U.S. Pat. No. 4,861,618).
With limited exceptions, ceramic thermal barrier coatings are applied to substrates which have previously received an intermediate, or bond coat. The bond coat provides enhanced adhesion and durability to the thermal barrier coating. The bond coat also affords a measure of environmental protection in the event that the ceramic thermal barrier coating is damaged in service.
Superalloys are alloys based on iron, nickel or cobalt having useful properties at temperatures in excess of 1,000.degree. F. Nickel superalloys are most widely used and Table I lists several exemplary nickel base superalloys.
TABLE I __________________________________________________________________________ (wt % Exemplary Superalloy Compositions) Cr Co W Cb Ti Al B Hf C Ni Ta Mo Zr Re __________________________________________________________________________ PWA1422 9 10 12 1 2 5 .015 1.6 .14 Bal -- -- -- -- PWA1426 6.4 12.6 6.4 -- -- 5.9 0.012 1.5 -- Bal 3.0 1.7 .08 .3 PWA1480 10 5 4 -- 1.5 5 -- -- -- Bal 12 -- -- -- IN 792 12 9 3.8 -- 4.1 3.5 .015 0.5 .12 Bal 3.9 1.9 .12 -- __________________________________________________________________________
Two major types of bond coats are used. The first type is the overlay or MCrAlY bond coat. MCrAlY bond coats are alloys of nickel or cobalt combined with chromium aluminum and yttrium. Other minor alloy elements including tantalum, platinum, rhodium, silicon, hafnium, rhenium and others are also possible. In service, the MCrAlY forms an oxide surface layer of relatively pure the alumina.
Table II lists several exemplary MCrMlY compositions.
TABLE II ______________________________________ (wt % Exemplary MCrAlY Compositions) Ni Co Cr Al Y Hf Si ______________________________________ NiCrAlY Bal -- 19.5 12.5 .45 -- -- CoCrAly -- Bal 18 11 .45 -- -- NiCoCrAl Bal 23 8 12.5 .3 -- -- NiCoCrAl Bal 22 17 12.5 .6 .25 .4 Y ______________________________________
MCrAlY overlay coatings have typical thicknesses of 75-175.mu..
The second type of bond coat is the diffusion aluminide bond coat, which are formed by diffusing aluminum into the surface of the superalloy substrate material to form a surface layer enriched in aluminum to ensure that the surface oxide which forms will be mainly alumina. Typical aluminide coating thicknesses are 30-70.mu.. A variety of aluminide coatings are known, a principal variant is the platinum aluminide in which a layer of platinum is applied to the substrate surface prior to the diffusion of aluminum. Platinum aluminide coatings have superior properties to normal aluminide coatings in certain applications. See U.S. Pat. No. 5,716,720 for a discussion of Pt aluminides as bond coats.
It is also possible to combine the aluminide and MCrAlY and overlay coatings (see for example U.S. Pat. Nos. 4,897,315, and 4,005,989).
Both MCrAlY overlay coating and aluminide coatings form an alumina layer of relatively high purity to which the ceramic thermal insulating layer will adhere.
The ceramic layer can be applied using physical vapor deposition techniques or by thermal spray techniques. Regardless of details of the bond coat and the ceramic application, the result is a gas turbine component comprising a superalloy substrate, a bond coat on the substrate, an alumina layer on the bond coat and a ceramic layer adhered to the bond coat.
The thermal barrier coated superalloy turbine component can be expected to survive for thousands of hours in a gas turbine engine with gases at temperatures as high as 2700.degree. F. flowing over the superalloy coated articles at a rate of as much as 1000 feet per second at pressures of as much as 400 psi. It will therefore be appreciated that the thermal barrier coating is durable and well adhered to the substrate.
Regardless of the initial durability, there will come a time in a life of a gas turbine component when repair or refurbishment of the coated part is necessary. It is then necessary to remove the ceramic coating and replace it with a new ceramic coating. Conceptually the ceramic can be removed mechanically or chemically, but the real problem is to remove the ceramic without removing or adversely affecting the bond coat. When the bond coat is removed or is chemically attacked, and has its surface composition altered, it is usually necessary to replace the bond coat, to ensure that the new ceramic coating will be adherent, and have the same durability as the original coating. This is a costly procedure which may not be economical.
Autoclaves are known in the superalloy industry, principally for use in removing ceramic molds and cores from superalloy castings. Shell molds can be 0.1-0.25 inches in thickness and cores may extend 3-6 inches inside coatings. When used to remove molds and cores, autoclaves are operated with caustic solutions of sodium hydroxide and/or sodium hydroxide at temperatures and pressures which cause aggressive attack and removal of the ceramic shells and cores in reasonable time periods. The emphasis in the prior use of autoclaves to remove shell and core ceramics from superalloys is on aggressive operation to remove large amounts of ceramics in reasonable times. The chemistry and quality of superalloy surfaces after the removal process is of little consequence because shell and core removal occurs early in the blade production process and many other steps will be performed which will shape and size the blade for its final function, and because protective coatings will subsequently be applied.
The prior art has also made some use of autoclaves and caustic solutions for blade cleaning, and for removal of thermal barrier coatings. See for example U.S. Pat. No. 5,685,917. These prior references of ceramic thermal barrier removal have been focused on certain features such as the use of special coolants to produce super critical fluid conditions in the autoclave process. The prior references have apparently not employed mechanical abrasion processing.
Accordingly, it is an object of the invention to disclose the process by which the ceramic portion of the thermal barrier coating can be removed without significantly changing the thickness of the bond coat or the chemical composition of the bond coat.
It is a further object of the invention to describe a thermal barrier coating refurbishment process.